Numerous products of commercial significance are formed of elastomeric compositions wherein particulate filler is dispersed in any of various synthetic elastomers, natural rubber or elastomer blends. Such products include, for example, vehicle tires wherein different elastomeric compositions may be used for the tread portion, side walls, wire skim and carcass. Other products include, for example, engine mount bushings, conveyor belts, windshield wipers and the like. It is well known that the performance properties of an elastomeric composition depend upon the elastomer or elastomer blend used in the composition, as well as the choice of particulate filler, additives and the method by which the composition is formed. While a wide range of performance characteristics can be achieved employing currently available materials and manufacturing techniques, there has been a long standing need in the industry to develop elastomeric compositions having improved formulations and properties and to reduce the cost and complexity of current manufacturing techniques. In particular, it is known for example that the macro-dispersion levels, that is, the uniformity of dispersion of particulate filler within an elastomer, can significantly impact performance characteristics. For elastomeric compositions prepared by intensively mixing the particulate filler with the elastomer, improved dispersion may require longer or more intensive mixing, with the consequent disadvantages of increased energy costs, manufacturing time, etc. In addition, particularly in the case of natural rubber, prolonged or more intensive mixing will degrade the elastomer by reducing its molecular weight, rendering the finished elastomeric compound undesirable for certain applications.
Carbon black is widely used as a reinforcing agent for natural rubber and other elastomers. It is common to produce a premixture of carbon black and elastomer, and various optical additives, such as extender oil, commonly referred to as masterbatch. Carbon black masterbatch is prepared with different grades of commercially available carbon black which vary both in surface area per unit weight and in "structure." It is well known to employ carbon blacks having higher or lower structure and surface area to manipulate the performance characteristics of an elastomeric composition. Carbon blacks of higher surface area and lower structure are known to improve abrasion and/or hysterisis properties, for example. It is difficult to achieve excellent uniformity of dispersion of extremely high surface area, low structure carbon blacks throughout the elastomer, however, without unacceptable degradation of the elastomer. In the highly energy consumptive intensive dry mixing methods mentioned above, the mastication of the elastomer necessary for dispersing the carbon black results in unacceptable levels of disruption of the polymeric chains of the elastomer. The resultant reduction in the molecular weight of the elastomer is undesirable for many industrial applications. For use in tire tread, for example, reduced elastomer molecular weight is known to cause an undesirable increase in the so-called rolling resistance of the tire.
Furthermore, while theoretical analysis has indicated desirable improvements in certain performance characteristics of carbon black masterbatch employing carbon blacks of higher surface area and lower structure, it has not been possible using known physical milling or other mastication processes to obtain such elastomeric compositions in which both the molecular weight distribution of the elastomer is preserved and satisfactory macro-dispersion levels are achieved. Generally, it has been found, for example, that the elastomer reinforcing properties of a carbon black increase as the particle size of the carbon black decreases. However, with extremely fine carbon blacks an anomalous condition is known to be encountered, in which the expected improvement in properties is not achieved. This is understood to be due at least in part to the inability of conventional elastomer compounding methods to adequately disperse the carbon black in the elastomer without undue breakdown of the elastomer polymer. There is consequent failure to take full advantage of the natural affinity of the carbon black and the elastomer for each other.
In addition to the dry mixing techniques described above, it also is conventional in synthetic elastomer latex masterbatching to continuously feed the latex and a carbon black slurry to an agitated coagulation tank. The coagulation tank contains a coagulant such as an aqueous acid and salt solution, typically having a pH of about 2.5 to 4. The latex and carbon black slurry are mixed and coagulated in the coagulation tank into small beads (typically a few millimeters in diameter) referred to as wet crumb. The crumb and acid effluent are separated, typically by means of a vibrating shaker screen or the like. The crumb is then dumped into a second agitated tank where it is washed to achieve a neutral or near neutral pH. Thereafter the crumb is subjected to additional vibrating screen and drying steps and the like.
Such methods present the disadvantage of a waste stream of used coagulant and the need to thoroughly wash the product crumb to remove acid, salt, uncoagulated latex and carbon black, etc. Such effluent streams cause both undesirable cost and manufacturing process complexity. There has, accordingly, been a long-standing need in the industry to reduce or eliminate such effluent streams.
Variations on this method have been suggested for the coagulation of natural and synthetic elastomers. In prior U.S. Pat. No. 4,029,633 to Hagopian et al, which like the present invention is assigned to Cabot Corporation, a continuous process for the preparation of elastomer masterbatch is suggested. An aqueous slurry of carbon black is prepared and mixed with a natural or synthetic elastomer latex. This mixture undergoes a so-called creaming operation, optionally using any of various known creaming agents. Following the creaming of the carbon black/latex mixture, it is subjected to a coagulation step. Specifically, the creamed carbon black/latex mixture is introduced as a single coherent stream into the core of a stream of coagulating liquor. The solid stream of creamed carbon black/latex mixture is said to undergo shearing and atomizing by the stream of coagulating liquor prior to coagulation, being then passed to a suitable reaction zone for completion of the coagulation. Following such coagulation step, the remainder of the process is substantially conventional, involving separation of the crumb from the waste product "serum" and washing and drying of the crumb. A somewhat similar process is suggested in U.S. Pat. No. 3,048,559 to Heller et al. An aqueous slurry of carbon black is continuously blended with a stream of natural or synthetic elastomer or latex. The two streams are mixed under conditions described as involving violent hydraulic turbulence and impact. As in the case of the Hagopian et al patent mentioned above, the combined stream of carbon black slurry and elastomer latex is subsequently coagulated by the addition of an acid or salt coagulant solution.
It is an object of the present invention to provide methods and apparatus for producing elastomeric compositions. In accordance with various preferred embodiments of the invention, it is a particular object to reduce or even eliminate some of the long standing disadvantages involved in conventional techniques used in the production of elastomer masterbatch. In particular, certain preferred embodiments have as an object the production of conventional elastomer masterbatch with significant reduction of the cost and complexity of the manufacturing process. Other preferred embodiments have as an object the production of novel elastomeric compositions having, in combination, desirable particulate loading levels and dispersion levels, better preservation of elastomeric molecular weight, use of high surface area, low structure carbon black or other particulate filler and/or other features not heretofore achievable using conventional production techniques. Additional objects and aspects of the present invention will be further understood from the following disclosure and detailed discussion.